1. Field of Invention
The present invention relates to a thermal head and to a printer that prints by selectively heating thermal paper that reacts to heat energy.
2. Description of Related Art
Thermal printers are a common type of printer. Thermal printers use a thermal head having small heating elements that selectively produce heat when energized arrayed in a line on a ceramic substrate. Thermal printers use a thermochromic printing technique to print on thermo-sensitive paper (thermal paper) by selectively melting dyes contained in the coating on the thermal paper. The thermal head used in this thermochromic printing method has a glazed layer that functions as a heat storage layer below the small heating elements as taught in Japanese Unexamined Patent Appl. Pub. JP-A-H07-137317. There are two types of glazed layers, a flat glazed type in which the glazed layer covers substantially the entire surface of the ceramic substrate, and a partially glazed type in which the glazed layer is formed only around the heating elements. The partially glazed type is widely used today because its high speed heat dissipation performance avoids the ghosting effect produced by residual heat in the heating elements during high speed printing when the heating elements are caused to respond rapidly.
The thermal paper has an undercoating applied to the base paper fiber matrix, and a thermochromic coating is then applied over the undercoating. In addition to improving the smoothness of the thermal paper, the undercoating also functions to cool the melted dye and fix the color.
However, thermal paper that does not have this undercoating is also widely used as a low cost thermal paper. With this type of paper the melted dye may separate before it can cool and be fixed, and printing chaff such as the melted dye then clings to and accumulates on the shoulder of the glazed layer adjacent to the heating elements. As this chaff accumulates, optical density, which is a measure of print density, drops as shown in FIG. 8, printing therefore becomes lighter, and print quality drops. Such printers are also preferably compact and as inexpensive as possible.